1. Field of the Invention
This invention relates to a blow molding process of a plastic product, and more particularly to a blow molding process in which a cooling is performed from the insides of a mold and a hollow molded product.
2. Description of the Invention
In a blow molding, after a sleeve-like molten resin called a parison extruded from an extruder is sandwiched between mold halves of a split mold, air is blown into the parison to inflate it so that a configuration of the mold cavity is applied to the inflated parison, and then the parison is cooled and hardened within the mold to obtain a desired hollow molded product.
In the blow molding, although the outside of a hollow molded product such as a bottle, or the like is cooled because it contacts the mold through which a cold water circulates, the inside of the hollow molded product is not cooled because the inside is in a heat insulated state. In case the inside of the hollow molded product is not cooled as just discussed, the pressure inside the hollow molded product becomes an original pressure of air to be introduced and the hollow molded product can be cooled by being intimately contacted with the mold. However, since only the cooling effect is obtained from the mold, cooling efficiency is bad.
A cooling method is disclosed in Japanese Patent Laid-Open Publication (Tokkai) No.3(1991)-13313, in which the pressure inside the hollow molded product is, in order to reduce the time for cooling, maintained at a prescribed pressure level and then air is discharged while blowing air into the interior of the hollow molded product, so that the hollow molded product is cooled also from the inside.
Japanese Patent Laid-Open Publication (Tokkai) No.3(1991)-222714 discloses a device in which an air blowing tube for introducing air, and a discharge tube for discharging air within a hollow molded product are, in order to enhance the inside cooling efficiency, located away from each other so that the air flows the entire hollow molded product. Further, Japanese Patent Laid-Open Publication (Tokkai) No.5(1993)-104615 discloses a device in which an air blowing port is rotated under air pressure so that the air is blown to the entire hollow molded product.
However, in case air is discharged while blowing air into the hollow molded product, the pressure within the hollow molded product becomes lower than the original pressure of the air. For this reason, the force for urging the parison against the mold becomes small compared with a case where no inside cooling is performed, and the cooling effect from the mold is lower than in the case where no inside cooling is performed.
That is, in the conventional inside cooling process, the cooling effect from the inside is available but on the other hand, the cooling effect from the mold is lowered. As a consequence, it is unable to obtain a sufficient cooling effect.